Resistance



March so, 1937. J HSHER' 2,075,515

RESISTANCE Filed Sept. 20, 1934 INVENTOR 5 7 J. R. F ASHE/Q J %(ac, M d

ATTORNEY Patented Mar. 30, 1937 UNITED STATES PATENT OFFICE RESISTANCEApplication September 20, 1934, Serial No. 744,780

6 Claims.

This invention relates to electrical resistance devices and moreparticularly to resistance devices of carbonaceous material.

The object of this invention is to provide a stable ohmic resistancedevice which is non-microphonic and which is suitable for grid leaks andother uses where the resistance value required is comparatively high andmust be stable at the operating potential.

A feature of this invention resides in a fibrous sheath of carbonsupported on an insulating rod.

Another feature resides in a method of producing the .fibrous sheath ofcarbon on the insulating rod and making contact between ends of thefibrous sheath of carbon and terminals on the rod.

Referring to the individual figures in the drawing:

Fig. 1 is a view in perspective of a sheath of braided threads;

Fig. 2 is a view in perspective of the sheath shown in Fig. 1 after ithas been partially processed;

Fig. 3 is a view in perspective of a rod of insulating material preparedto receive the sheath shown in Fig. 2;

Fig. 4 shows the sheath of Fig. 2 supported on the rod of Fig. 3;

Fig. 5 shows the sheath and rod of Fig. 4 after the sheath has beenfurther processed than as shown in Fig. 2; and

Fig. 6 shows the finished resistance.

I have found in resistance devices made of finely divided carbonaceousmaterial and a binder that such resistance devices almost invariablycontain incomplete paths of the carbonaceous material. These incompletepaths are frequently arced by the current traversing the resistance andthe resistance device, therefore, ,is microphonic. The microphoniccharacteristic of the resistance device, therefore, is

' due to there being in the resistance device incomplete paths for thecurrent applied and the arcing-over of the gaps in these paths by thecurrent traversing the resistance. For instance, in resistance devicesmade of carbon granules held in required form by a binding material,there are numerous places in the resistance device where the carbongranules do not touch adjacent carbon granules, but are slightlyseparated therefrom by some of the binder material. In resistancedevices of the grid leak type where the resistance path is formed byapplying a paint or India ink to a strip of insulating material and inresistance devices in which the resistance path is formed by rubbing orotherwise applying graphite or other resistance material to a rod ofinsulating material, small gaps between particles of the resistancematerial are almost invariably found to be present in the resistancedevice.

In this invention due to the character of the material in the resistanceelement, and due to the method employed in making the resistance, acomparatively large number of unbroken paths of resistance material areproduced from end to end of the device.

In the drawing in which like parts bear like numerals, I is a sheath ofbraided threads in which each thread is continuous from one end of thesheath to-the other. The sheath I may be made of braided cotton threads,or threads of other suitable material and may be cut to a predeterminedlength from a comparatively long piece of sheathing. The sheath I afterbeing cut to the required length is parchmentized by applying a. solventsuch, for instance, as sulphuric acid to the sheath suflicient topartially dissolve the threads. After washing, to check furtherdissolution of the threads, the sheath is dried. The sheath may be driedin air or in any other suitable drying atmosphere. I have found thatwhen the sheath I is subjected to the application of sulphuric acidsuificient to partially dissolve the threads, the sheath becomes of ajelly-like consistency. When the sheath is dry it has shrunk about 10%in length and diameter. The sheath I, therefore, shown in Fig. 1 becomesthe parchmentized sheath 2 shown in Fig. 2. The braided cotton threadsin the sheath I in Fig. 1 as before mentioned, are unbroken from end toend of the sheath and are still unbroken in the sheath 2 of Fig. 2, buthave become smaller in diameter and smaller in length in Fig, 2 thanthey are in Fig. 1.

As shown in Fig. 3, a rod 3 of insulating material is prepared toreceive the parchmentized sheath 2 of Fig. 2. The rod 3 may be, forinstance, a glass rod of suitable diameter and length to receive theparchmentized sheath 2. Terminals 4 of conducting material are providedon the ends of the rod 3. The terminals 4 may be formed and applied byspraying the ends of the rod 3 with particles of finely divided metaluntil a coating of metal 5 is formed on the ends of the rod 3. Anotherway in which the ends of the rod 3 may be metallized is to rub asuitable metal compound on the ends of the rod 3 until the requiredcoating of metal is obtained.

To provide means for connecting the resistance tion of the threads.

device into a circuit, conductors are attached to the metallized ends ofthe rod. oi the conductors lized ends of th rod After the terminals f,comprising the metallized ends 5 of the rod and the conductors have beenapplied to the rod the parchmentized sheath 2 of Fig. 2 is placed on therod as shown in 4 with the ends of the sheath extending over themetailized ends i of the rod 3.

The rod 3 and the sheath 2, assembled as shown in Fig. are then placedin an oven having therein an atmosphere of hydrogen and are baked in theoven and in the atmosphere oi hydrogen under a suitable temperature andfor a suidcient length of time to reduce the threads in theparchmentized sheath 2 to carbon. 3 have found that by baking theparchmentized sheath in an oven in an atmosphere of hydrogen that gasesreleased from the parchmentized sheath in the baking process are quicklytaken up by the hydrogen and that there is no localization of the gasesreleased such as might cause explosion in the threads of the sheath 2and breaking or disrup- The parchmentized sheath 2, therefore, under thebaking process is not broken or disrupted and the threads remain intactand provide threads of carbon which are unbroken from end to end of theresistance device.

The baking process as to length or time and heat applied may be variedto obtain different resistance values. The heating of the oven shouldalso be regulated to prevent burning of the parchmentized material andto permit taking up by the atmosphere of hydrogen of the gases releasedirom the parchmentized material. I have found for instance that theparchmentized material begins to turn to carbon at a temperature ofabout 300 centigrade and that the oven con-- taining the device, underheat treatment, should be gradually brought to about this temperature topermit the gases released from the parchmentized material to be taken upby the hydrogen. After the oven has been gradually brought to acarbonizing temperature the heat of the oven may be more rapidlyincreased to a required predetermined higher temperature. For instanceto obtain a resistance of 1000 ohms a parchmentized element ofpredetermined length, texture and diameter is placed in the oven. Thetemperature of the oven is then brought from room temperature to about300 centigrade in from five to ten hours. The temperature of the oven isthen further increased to about 700 centlgrade in about two hours fromthe time the 300 centigrade temperature was obtained and the elementunder heat treatment is left in the oven at about this temperature forabout one-half hour. The oven with the element under heat treatment isthen permitted to gradually cool to room temperature. I have also foundthat if the temperature of the oven is raised much above 1100 centigradethe element under heat treatment becomes burned and the threads will bedestroyed.

During the baking process the sheath 2, as before mentioned, becomescarbonized and shrinks in length and diameter on the rod as shovm inFig. 5, and becomes the carbonized sheath which, as shown in Fig. 5, issmaller in length and diameter than the sheath 2 in Fig. The sheath inthe baking process also shrinks against the metallized ends 5 of the rod3 and makes intimate contact with the metallized ends or" the rod.

To protect the resistance device from moisture and other injuriousagencies the unit as assembled, as shown in Fig. 5, may be dipped inparaf-' fin or in some other suitable Water-proofing compound or may bepainted or otherwise provided with a moisture-proof coating 0 as shownin What is claimed is:

1. An electrical resistance device comprising a rod of insulatingmaterial, conducting terminals on the ends of the rod, a textile sheathof Woven textile carbon threads shrunken the rod and end portions ofsaid sheath in engagement with said conducting terminals.

2. An electrical resistance device comprising a rod of insulatingmaterial, conducting terminals on the ends of said rod, a woven textilesheath of unbroken carbon threads shrunk on said rod and end portions ofsaid woven textile sheath in engagement with said conducting terminals.

3. An electrical resistance device comprising a rod of insulatingmaterial, metallized portions on the ends of said rod, and a woventextile sheath of unbroken carbon threads shrunk on and enclosing saidrod and in contact with the metallized portions on the ends of the rod.

4. A method of producing an electrical resistance which comprisesapplying conducting material to ends of an insulating rod, placing asheath of woven textile over the rod, parchmentizing the sheath andsubjecting the sheath and rod, in an atmosphere of hydrogen, tocarbonizing tern perature for the sheath of woven textile to reduce thesheath of woven textile to carbon and to shrink the sheath intointimatecontact with the conducting material on the ends of the rod.

5. An electrical resistance device comprising a tube of insulationhaving its ends provided with a coating of conducting material and atextile sheathing slipped over the tube and parchmentized and carbonizedin place so that the sheathing shrinks into intimate contact with themetallized ends of the tube.

6. A method of producing an electrical resistance device comprisingapplying conducting material to ends of an insulating rod, placing abraided sheath of cotton threads on the rod, partially dissolving thecotton threads, drying the threads, placing the rod and sheath in aninert atmosphere containing hydrogen, raising the temperature of thesheath in about ten hours time to 300 centigrade, increasing thetemperature in about two hours time to 700 centigrade, maintaining thesheath at the higher temperature for about one-half hour and thenallowing the sheath to gradually cool in the atmosphere containing thehydrogen.

JOSEPH R. FISHER.

